I'm fairly confident that schema updates are always backwards-compatible. The only problem would stem from an application that checks for the domain level as an 'equals' ("=") rather than a 'greater-than-or-equal' ("<=") (as in 'domain level is 2003 native' rather than 'at least 2003').
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gWaldoSep 10 '10 at 19:40

@gwaldo It's schema version, not Functional Level. The Schema gets updated the moment the right 'adprep' call is made, so you can have a 2008-level schema but still 2003 functional level. If @Knox is concerned about schema issues, functional level doesn't matter.
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sysadmin1138♦Sep 10 '10 at 19:44

well, I was pretty sure that nothing would care if I did upgrade the schema, but if I could avoid it until I actually needed, I would probably hold off. We don't run any apps other than Windows clients that are aware of AD so fortunately it's very low risk to upgrade the schema.
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KnoxSep 11 '10 at 0:50